Okay, big brain here, step aside…! This one I have wasted more hours of my life than I care to consider on trying to find the answer to this one!!!
The answer is…I don't know! Nobody does! It's a giant secret that the writers refuse to tell anyone!
Okay, not quite true, twill the first part is. There are no settings to change for it, the theory is (and the stock answer from the phpBB forum Mods) that it should show all posts since you last logged in, then clear the list.
The truth of the matter seems a little different with pages and pages of posts of people reporting seeing anything from only the last hours worth, to 3 days worth, with some people even claiming that it shows some from 2 hours BEFORE they last logged in. It also seems to adjust the list while you are still logged in, which it's not really supposed to do from what I can see.
So the truth of the matter is, there are no setting so change it, it
should show you any and all posts since you last logged in, and it appears to be buggy and do whatever the hell it likes!
HTH
Cheers
Mark